The present invention relates to an ellipsometer and is a technology of measuring variation in polarization state which a light incident to a surface of a specimen with a specific polarization state comes to have after being reflected and analyzing the measured value, thereby finding out optical physical properties of the specimen. Particularly, a variety of nano-thin film manufacturing processes is used in semiconductor industries, and an ellipsometer which is non-destructive and contactless real-time measuring equipment is widely used as a measuring equipment to evaluate physical properties of the manufactured thin films.
A variety of ellipsometers is used not only in the semiconductor industries but also in research institutions such as a university, and is classified, according to how the light used as a probe is incident to the specimen, into a type in that a parallel light is incident to a specimen in an inclined direction (U.S. Pat. No. 3,985,447), a type in that a light is focused onto a specimen in an inclined direction using an optical system (U.S. Pat. No. 5,166,752 and U.S. Pat. No. 5,608,526), and a type in that a light is focused onto a specimen in a vertical direction using an optical system (U.S. Pat. No. 5,042,951 and U.S. Pat. No. 6,898,537).
In the case that a parallel light is incident in an inclined direction, the ellipsometer has excellent measuring accuracy since it is possible to accurately control an angle of incidence as all light rays are irradiated onto the specimen with the same incidence angle, but it is difficult to reduce a size of a beam incident to the specimen to less than a mm since diffraction is increased when reducing the size of the incident beam using an iris.
In semiconductor industries, a wafer particularly provided with a measuring region limited in an area of tens μm×tens μm is used to evaluate the variety of thin film manufacturing processes for the manufacture of semiconductor devices through the measurement. In order to measure the thickness of the thin film within the measuring region limited in the micrometer level using an ellipsometer, a technology of focusing a parallel light incident in an inclined direction on the surface of a specimen using an optical system consisting of a lens or a reflective mirror is used. However, in this case, it is difficult to ensure and maintain the measuring accuracy compared to using the parallel in an inclined direction since light rays having a plurality of incidence angles are incident to the specimen at the same time.
As a critical dimension patterned in a wafer is expected to be continuously reduced with recent continuous development in a semiconductor device manufacturing technology, the area of the limited measuring region have to be correspondingly reduced. However, in the case of the inclined directional focused-beam ellipsometer, it is actually difficult to reduce the size any more due to barrier such as aberration and structural limitation of the focused-beam optical system despite of many studies and efforts for reducing the area of the light beam irradiated onto the specimen as small as possible.
The vertical-incident focused-beam ellipsometer permits the measurement within a fine pattern region having smaller area since it is able to focus a parallel light incident in a vertical direction onto the surface of the specimen in a size of several μm using an objective lens.
FIG. 1 shows a basic structure of the vertical-incident focused-beam ellipsometer (U.S. Pat. No. 5,042,951) which is currently widely used. The focused-beam ellipsometer is provided with a polarization generator 130 for making a parallel light 120 emitted from a light source 110 into a specific polarization state, a non-polarizing beam splitter 140 for splitting some of the light passed through the polarization generator 130, an objective lens 150 for refracting and concentrately irradiating the light split by the non-polarizing beam splitter 140 onto a specimen 160, a polarization detector 170 for filtering only the specific polarization state from the light passed through the objective lens 150 and the non-polarizing beam splitter 140 after reflected from the specimen 160, and a photodetector 180 having pixels for detecting an intensity of the light passed through the polarization detector 170.
Meanwhile, U.S. Pat. No. 6,898,537 suggests that by placing a single linear polarizer, instead of the polarization generator 130 and the polarization detector 170 of the conventional focused-beam ellipsometer, is installed in the non-polarizing beam splitter 140 and objective lens 150 and rotated by a motor, and a spectroscopic photodetector is employed as the photodetector, and it is therefore possible to analyze the light refracted by patterned specimen.
Such the conventional patent technology has a relatively complex structure and necessarily requires a calibration process for finding out an optical axes of the polarization generator 130 and the polarization detector 170 with respect to the reflection face of the non-polarizing beam splitter 140 for the accurate measurement, since the polarization generator 130, the non-polarizing beam splitter 140 and the polarization detector 170 are separately installed as shown in FIG. 1 or some optical component can be rotated.